1. Field of the Invention
The invention pertains to the art of shaft, plate and hub assemblies for hot gas fans such as those used for induced draft and gas recirculation duty.
2. Fan Design Considerations and Description of the Prior Art
Before discussing two prior art patents which may superficially seem to be quite pertinent to this invention, some design considerations in connection with centrifugal fans which operate in high temperature environments are believed worthy of discussion. A fan plate operating in a hot gas environment, such as at 750.degree. F. (399.degree. C.), and where the operating temperature has been reached from normal ambient temperature in a period of about 30 minutes, has a temperature gradient from the outer periphery of the plate to the inner periphery of the plate which ranges from about 590.degree. F. (310.degree. C.) at the outer edge to about 200.degree. F. (93.degree. C.) at the inner periphery adjacent the shaft. This condition occurs with a fan construction in which the plate is attached to the shaft by a pair of opposing hub members, one of which is integral with the shaft and the other is in the form of a ring. As a result, the stresses in the plate successively increase from the radially outer parts of the plate toward the location of connection of the plate to the hub. This condition of the stresses imposed upon the plate by the thermal transient may be visualized as being analogous to a simple disc which has its radially outer portion subjected to heat, while the radially inner portion is not heated or is subjected to cold. The outer portion under these conditions will grow and be in strain while the radially inner portion of the disc will not correspondingly grow so that large stresses are set up between the radially outer portion and the radially inner portion of the disc. It is such stress to which the bolts and the centerplate and hub material is subjected, along with the centrifugal force stress of course.
Taking care of stresses imposed upon the bolts poses no great problem since high quality bolt material is available, and is relatively inexpensive in the sense that not much bolt material is required. However, it will be apparent that using equally high quality material for the fan plate, which is of course many, many times larger, is not practical. Accordingly, it would be desirable to provide a shaft, hub, plate assembly in which the stresses resulting from rapid temperature excursions experienced in high temperature applications of the centrifugal fan can be survived with no ill effect to the fan.
Two patents relating to high temperature centrifugal fan hub assembly constructions are U.S. Pat. Nos. 3,602,609 and 3,832,091. In the first, the shaft is comprised of two stub shafts secured to the opposite sides of a centerplate (which is devoid of a center hole), with each stub shaft end adjacent to the centerplate being enlarged and hollowed to form enlarged diameter ring-shaped shaft end mounting flanges to provide a ring of thermal and mechanical contact at the centerplate, and with each side of the centerplate being provided with a countersunk ring surface in which the shaft flanges are seated, and with fastening bolts extending through the flanges and the thinner portion of the centerplate. In the latter patent, the centerplate has a central opening with a boss at its inner periphery received in annular recesses of the opposing hub members, and with the hub bolts being radially outwardly therefrom and extending through the hub members and the centerplate at a normal thickness location. While both of these superficially are pertinent to this patent application, it is believed that with the foregoing design considerations in mind and the description hereafter, the superficial pertinency will be found to be without substance.